The present invention relates to a high-performance X-ray generating target. More particularly, the invention is directed to a high-performance rotating X-ray tube anode structure having an improved target and a related method of manufacturing such an anode structure.
X-rays are produced when electrons are released in a vacuum within an X-ray tube, accelerated and then abruptly stopped. The electrons are initially released from a heated, incandescent filament. A high voltage between an anode and a cathode accelerates the electrons and causes them to impinge upon the anode. The anode, usually referred to as the target, can be a rotating disc type so that the electron beam constantly strikes a different point on the target surface. The X-ray tube contains the cathode and anode assembly, which includes the rotating disc target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the X-ray tube vacuum envelope, overlapping about two-thirds of the rotor. The X-ray tube is enclosed in a protective casing having a window for the X-rays that are generated to escape the tube. The casing is filled with oil to absorb heat produced by the X-rays.
The rotating X-ray tube target typically includes a refractory metal target substrate and a target focal track of an X-ray emitting metal joined to the target substrate along an interface. Tungsten alone and tungsten alloyed with rhenium are commonly used to form the focal track in X-ray targets. X-ray targets formed wholly from tungsten or from tungsten alloys, wherein tungsten is the predominant metal, are characterized by high density and weight. Additionally, tungsten is notch sensitive and extremely brittle and is thereby subject to catastrophic failure. Because of these shortcomings, X-ray targets typically comprise a tungsten or tungsten alloy target focal track and a target substrate of another metal or alloy. Typically, molybdenum and molybdenum alloys are used for the target substrate.
X-ray tubes used for medical imaging generate X-rays by bombarding the layer of material making up the target focal track with high-power electrons. The focal track contains elements with high atomic number (such as tungsten and rhenium) and is integrally attached to a disc of a high-conductivity refractory metallic material such as TZM (a molybdenum alloy containing small amounts of titanium, zirconium and carbon). The TZM alloy disc in turn is bonded onto a graphite disc by a braze layer composed of titanium, vanadium or zirconium alloys. In order to dissipate the intense heat generated on the focal track, the target disc is rotated to speeds in excess of 8,400 rpm. Additionally, the high-conductivity target disc conducts the heat generated under the focal track to the brazed graphite block, which acts as thermal storage material or a heat sink.
The demand for ever-improving X-ray image quality in conjunction with the need for computerized tomography (CT) systems to perform high-speed cardiac imaging necessitates the use of high peak power (in excess of 70 kW), high target rotation speeds, as well as high gantry rotation speeds. These in turn drive up the thermal and structural loading of the target material beyond its current capabilities. Thus, there is a need for target materials with (a) higher strength and creep resistance than those for the TZM alloy to meet the thermal and structural demands placed by the use of high peak power and high rotation speeds, and (b) lower target weight compared to the current TZM/brazed graphite configuration to offset the impact of higher g-loads at faster gantry speeds on bearing stresses.
Efforts to address these requirements in the past have included the potential use of targets made of carbon-carbon composite materials. While these materials offer substantial advantages in terms of weight savings and thermal storage, they also have inherent drawbacks, namely their limited toughness. In addition, carbon-carbon composite materials have issues with fabricability, burst strength vacuum compatibility, and material homogeneity. Consequently, their implementation in CT X-ray systems is still under development.
The present invention is directed to an X-ray target material for use in rotating anode X-ray tubes in which the TZM material used in current X-ray targets is replaced with an oxide-dispersion strengthened Mo (ODS-Mo) alloy. ODS-Mo refers to molybdenum strengthened by a fine dispersion of insoluble oxide particles of one or more of the following compounds: La2O3, Y2O3 and CeO2.
One aspect of the invention is an X-ray tube anode comprising a target substrate made of oxide-dispersion strengthened molybdenum alloy, a metal track formed on the target substrate and comprising X-ray emitting metal, a graphite mass brazed on the rear of the substrate, and an emissive coating applied to open ODS-Mo surfaces.
Another aspect of the invention is an apparatus comprising a substrate made of oxide-dispersion strengthened molybdenum alloy and a metal track formed on the substrate and comprising X-ray emitting metal, wherein the substrate has a generally circular outer periphery and a central hole, and the track is generally annular and concentric with the outer periphery of the substrate.
A further aspect of the invention is a method of manufacturing an X-ray tube anode, comprising the following steps: extruding molybdenum powder alloyed with dispersed oxide to form a workpiece; upset forging the workpiece to form a target substrate in the shape of a circular disc with a circular cylindrical shaft attachment projecting from the periphery of a central hole in the disc; and coating an annular section on one side of the target substrate with a layer of X-ray emitting metal.
A further aspect of the invention is a method of manufacturing an X-ray tube anode, comprising the following steps: extruding molybdenum powder alloyed with dispersed oxide to form a workpiece; plate rolling to more than 92% cross-section reduction followed by cutting of right circular discs from the plate; and coating an annular section on one side of the target with a layer of X-ray emitting metal.
Yet another aspect of the invention is an anode assembly for an X-ray tube, comprising a rotating disc target and a rotor that is part of a motor assembly that spins the target, wherein the disc target comprises a target substrate made of oxide-dispersion strengthened molybdenum alloy, a metal track formed on the target substrate and comprising X-ray emitting metal, a graphite mass brazed to the rear of the substrate, and an emissive coating applied to open ODS-Mo surfaces.
Other aspects of the invention are disclosed and claimed below.